1. Field of the Invention
The present invention relates to a link adaptation method in an Automatic Repeat request (ARQ) System, and more particularly to a method of a link adaptation of a blind type using power offset and multi-codes transmission etc. through acknowledgements (ACK/NAK) sent to a transmitting party based on the result of the quality judgment of a receiving signal in a receiving party.
The present invention relates, still more particularly, to a method of the link adaptation to improve a system performance in a transmitter by gradually controlling the number of multi-codes transmission and transmission power with a fixed type power offset and multi-codes transmission according to the retransmission request transmitted, without additional control signals or control channels at a uplink that has been established between a receiver and a base station, when a signal (NAK response) required for packet retransmission from the receiver to a transmitter exists in the ARQ system including a Hybrid type.
2. Description of the Related Art
First of all, the necessity and a related method for controlling power adopted in a Code Division Multiple Access (hereinafter referred to as CDMA) system will be described as a link adaptation method.
In the CDMA system, if necessary, more than two multi-codes can be assigned to a user. This is intended to realize the higher transmission ratio of data than in a case where only one code is used. In the future, a requested data rate will likely be up to 10 Mbps in downlink and up to 20 multi-codes will be available in the art. But, if the same data is transmitted to the multi-codes assigned as the purpose, an effect of controlling transmission power can be obtained.
Closed and fast power control is the most important factor in the CDMA system, because if the closed and fast power control is not properly performed, an over-powered mobile station can interfere the communication within the entire cell.
The power control is to adjust the transmitting power of a mobile station and a base station in order to maintain the system performance even with the lowest transmitted power level.
There are two kinds of power control: a forward link power control and a reverse link power control.
The forward link power control controls the transmitting power of the base station in order to reduce the interference to adjacent cells.
Meanwhile, the reverse link power control controls the transmitting power of the mobile station in order to resolve Near-Far Interference Problems, which is resulted as all of the mobile stations in a cell use the same frequency range.
Generally speaking, when considering a path loss resulted from a wireless link between the base station and the mobile station in mobile communication environment, it is known that the amplitude of radio wave that is influenced by the distance is diminished. Accordingly, if the power of the mobile station were constant, the receiving power from the mobile station located far away from the base station would be small compare to the mobile station located adjacent to the base station.
Because of the capacity in the CDMA system is maximized when the receiving power of each mobile station is the same, the CDMA capacity can be seriously diminished in the above situation. That is to say, it is impossible for a base station to demodulate a signal from a mobile station far away from the base station regardless of despreading, because it has smaller signals than the interference caused by an adjacent mobile station.
This is generally called as ‘Near and Far Problem’. In order to overcome the ‘Near and Far Problem’, the transmitting power of the mobile station should be adjusted such that the receiving power of each mobile station in a base station may be constant. That is to say, the mobile station adjacent to the base station should transmit the signal in a lower transmitting power, and the mobile station far away from the base station should send out the signal in a higher transmitting power. This is called ‘power control’, and very accurate and elaborate power control system is needed for the CDMA system.
Accordingly, the best idea in light of the maximum capacity is that the bit power received from all the mobile stations should be constant.
In addition to ‘Near and Far Problem’ that shows the big difference in signal power received by the base station as a result of the distance between the mobile stations and the base station, there is fading in each mobile station. Therefore, in order to overcome this environmental obstacle and maximize the capacity for subscribers, an elaborate power control with a broadly operating dynamic range is required for a reverse link.
In order to maximize the capacity of the CDMA system, the signal of each mobile station should be received with a minimum Signal to Interference Ratio (SIR) in the base station. If the transmitting power of the mobile station is low, traffic quality becomes low, while if the transmitting power of the mobile station is high, the traffic quality becomes high. But other mobile stations will be effected with much interference in the same channel and the traffic quality of other subscribers will be deteriorated. Accordingly, in order for all of the subscribers to maintain a good traffic quality and maximize the capacity, the transmitting power of each mobile station received at the base station should be the same and its value should be the minimum Signal to Interference Ratio by controlling the transmitting power of each mobile station.
As described above, the power control can be described with the forward link power control and the reverse link power control.
There are an open loop power control and a closed power control in the reverse link power control.
In a mobile station, it is really difficult to predict the exact path loss of reverse channel because of the fading characteristics from a different frequency range and mismatch between a receiver and a transmitter, and the difference between the forward channel and the reverse one. In order to correct this error, each mobile station needs to control outputs by the power control command in a low speed from the base station through the forward channel.
The base station acquires error correction information by monitoring the state of the reverse channel and comparing with a preset value, and gives a command to the mobile station for either to increase or to decrease the outputs based on the comparison results. With such method, the base station can simultaneously meet an appropriate traffic quality and capacity maximization by controlling the power of the reverse channel of every mobile station.
Meanwhile, there are an open loop power control and a closed loop power control in the forward link power control.
If the forward link is defective, the traffic quality doesn't reduce to below the standard level when the transmitting power of the base station increases. For example, if the mobile station is in an area where there is a similarity in the path loss for both the current calling base station and the adjacent base station in a cell boundary area having two or three cells folded, (or an area where there is serious path loss due to fading and source of strong interference, etc), this mobile station needs to increase the transmitting power of the base station for the deteriorated traffic quality resulted from the interference by other adjacent base stations.
In contrast, in case that the mobile station is near the base station and Signal to Interference Ratio is in a good state, the interference to other mobile stations can be diminished by reducing the transmitting output of the base station so as not to seriously affect the traffic quality for the traffic channel.
1. Reverse Link Open Loop Power Control
Each mobile station measures the total received power of all CDMA channels in a specific base station. Since the entire power is monitored without demodulated signals, it is rather easy to predict the power without any information about synchronization time, the name of the base station, and the path loss.
The mobile station transmits the average outputs calculated by an initial searching. L the following procedure of an access search, the output is increased up to the level of the time when the corresponding response exists. After initializing the reverse traffic channel with the average transmitting output of an initial reverse traffic channel going through this process, if the output control bit is received from the mobile station, the power control mode is switched into the closed loop power control.
2. Reverse Link Closed Power Control
In the power control procedure, an Eb/No prediction value is measured at the base station with a period of fixed time interval, and compared with a preset limit Eb/No value, and transmitted a corresponding command to the mobile station every interval.
Here, the variation quantity of average output is 1 dB per power control bit. The mobile station controls the power of the closed loop within the range of greater than 24 dB ± of the open loop measurement value; an upper limited value is determined by a maximum output.
3. Forward Link Open Loop Power Control
In the forward link open loop power control procedure, the base station forecasts the forward link loss based on the received power of the mobile station, controls the initial digital gain of each traffic channel with the forecasted value, and allocates a standard gain for each channel at the initial stage.
4. Forward Link Closed Loop Power Control
In the forward link closed loop power control, the mobile station measures the quality of forward traffic channel frame and reports to the base station periodically, so that the base station can compare the value with a preset value, and adjusts the output of the forward traffic channel accordingly. And the mobile station automatically reports the value to the base station in case where the number of defective frame exceeds the preset standard value, and the base station raises the outputs allocated to each channel. Also, all of the mobile stations maintain the traffic quality of the forward traffic channel through this procedure, and the base station has an additional function in order to not to reach the saturation state of a power amplification.
In the CDMA system, a major power control method is the closed loop power control as described above.
FIG. 1 shows the closed loop power control in the CDMA. When a mobile station MS1 and anther mobile station MS2 operate with different spreading codes at the same frequency, the mobile station MS1 which located near the outer boundary of a cell experiences the path loss of about 70 dB lower than that of the mobile station MS2 located adjacent to a base station BS. If the mechanism of the base station BS that controls the power at the same level doesn't exist for the far away mobile station MS1 and the near by mobile station MS2, the mobile station MS2 located near by the base station BS blocks many parts of a cell by maintaining larger power than that of the mobile station MS1 located near the outer boundary of a cell and far away from the base station. As previously described, this problem is known as ‘Near and Far Problem’.
The Near and Far Problem is one of chronic problems in the CDMA system in which the channel capacity of the system being basically determined by the interference, because of the channel capacity is affected by the wave characteristic of an electromagnetic wave.
As shown in FIG. 1, the base station BS frequently measures Signal to inference Ration (SIR) and compares with a target SIR in the closed loop power control of uplink 101. If the measured SIR is higher than the target SIR, the base station sends out commands prescribing that the mobile station should lower the power. But, if the measured SIR is much lower, the base station sends out commands that the mobile station should increase the power. A series of procedures including measurement, command, and response, are performed 1500 times (1.5 kHz) per second for each mobile station and the speed gets faster than the change of path loss which sometimes occurs, and even faster than Rayleigh fading at a lower or middle moving speed. Therefore, the power unevenness among all the uplink signals received from the base station can be resolved.
In the same manner, the closed loop power control can be operated in the downlink 102 as described above. Since 1:n(n≧2) communication is performed in the downlink, the ‘Near and Far problem’ doesn't occur. All the signals in a cell are transmitted from the base station to all of the mobile stations, therefore, it is desirable to provide a minimum additional power to the mobile station located adjacent to the boundary line. The reason is that other cell interference can be increased. Also, it is required to intensify a weakened signal of low speed due to Rayleigh fading in the downlink because interleaving and an error correction code method does not work effectively.
As described above, the fast closed loop power control is called ‘inner loop power control’ and it is an essential operation due to uplink Near and Far Problem in the CDMA system. The fast closed loop power control operates at the uplink and the downlink at the speed of 1500 Hz based on a command per a slot.
For reference, GSM supports slow power control (2 Hz), and IS95 supports 800 HZ power control only in the uplink. The basic step size of power control is 1 dB. Additional plural step size can be used and much smaller step size can be performed after modification. Small step size means that 1 dB is operated at every two slots, so that it has the same effect as that of 0.5 dB operation. Actually, the operation below 1 dB results in complexity problem. The gain is greater in slower moving speed than it is in fast moving speed, request Eb/No rather than the transmission power and the case there are only a few usable multi-paths such as ITU Pedestrian A channel.
Meanwhile, as described above, there is the open loop power control method as the power control method in view of Near and Far Problem and the maximum acceptance capacity, and the method is not really precise one since it roughly estimates the path loss by using the downlink bit control signal.
The reason is that the fast fading has no correlation between the uplink and the downlink due to separation of a frequency band. However, the open loop power control is used when the initial power setting for the mobile station is roughly set at the time when a call is established.
The open loop control is performed before RACH or CPCH transmission is initialized. However, the adjustment is not accurate, because it is difficult to measure the large variation of the power in the mobile station. The requirement for accuracy is within ±9 dB.
FIG. 2 is a view showing the outer loop power control. The outer loop power control targets the request of individual wireless link in the base station BS and the quality of a preset level defined as BER and FER, and makes correction for the target SIR. An optimum plan is to operate the target SIR value near the minimum value to meet the request target quality.
FIG. 3 is a view showing a procedure of the general outer loop power control. The target SIR can be decreased or increased depending on the comparison result after comparing the received quality with the target quality.
The outer loop power control is required to maintain communication quality required level by setting the target value of fast power control (target SIR).
The outer loop power control is required for the both uplink and the downlink because the fast power control is performed at both links.
For reference, in IS-95 system, the outer loop power control is used only in the uplink because there is no fast power control in the downlink. The fast power control frequency is 1.5 kHz and the outer loop power control frequency is 10˜100 Hz. The down link outer loop power control based on a network increases signaling load between the mobile station and the radio network controller (RNC) and results in the time delay. Accordingly, the outer loop power control based on the mobile station is used in the WCDMA.
The related fast closed loop power control can produce rather poor performance than slow open loop power control because of SIR assumption error, the power control signaling error and the power control loop time delay. Especially, in order to maximize the acceptance capacity of downlink which should support high data rate in the CDMA, the transmission power control required for each link should be performed. However, in some cases, it is difficult to perform the additional power control, thus another type of power control is preferred than the direct transmission power control.